1. Field of the Invention
The present invention relates to an input protection circuit, more particularly to an input protection circuit preventing electrostatic discharge damage of a semiconductor integrated circuit.
2. Description of the Related Art
A semiconductor integrated circuit has gate input circuits, such as field effect transistors, as input/output terminals. The gate input circuits connected to the input/output terminals are sometimes destroyed by electrostatic discharge (ESD) from the human body or various devices. Particularly, due to the ESD applied between the input/output terminals before mounting semiconductor integrated circuits on a circuit board or the like, gates of a field effect transistor used for an input circuit are sometimes destroyed.
As a measure to solve the above problem, disclosed is an input protection circuit in which voltage levels of input/output terminals do not exceed a supply voltage because of diodes (hereinafter referred to as “the first conventional technology”). The input protection circuit according to the first conventional technology includes a diode having its cathode terminal connected to a power source and its anode terminal connected to an input/output terminal and a diode having its cathode terminal connected to the ground and its anode terminal connected to the input/output terminal. According to the first conventional technology, when the ESD is applied to the input/output terminal, a current path to the power source or the ground is created by diode forward conduction or reverse breakdown. Thus, application of a high voltage to internal elements of a semiconductor integrated circuit can be prevented.
Moreover, disclosed is an input protection circuit in which fuses are provided between input/output terminals and diodes (hereinafter referred to as “the second conventional technology”). In the input protection circuit according to the second conventional technology, when a direct current is applied to a fuse after a semiconductor integrated circuit is mounted, the fuse is blown out. When the fuse is blown out, the input protection circuit is separated from the input/output terminals. Thus, the input protection circuit is not affected by a parasitic capacitance that the diodes have. When there is no influence of the parasitic capacitance of the diodes, a high-speed transmission of an input signal is not impaired.
However, a junction area of the diodes of the input protection circuit according to the first conventional technology is designed to be about several square micrometers in order to instantly apply a large amount of charges associated with high-voltage electrostatic discharge. In this case, a junction capacitance of the diodes sometimes reaches several pF. There was a problem that attenuation of a signal level and a propagation delay of a signal occur due to a parasitic capacitance of the diodes and the like.
Moreover, the input protection circuit according to the second conventional technology has the diode in a cell. Thus, there was a problem that the diode in the cell causes an increase in an area of a semiconductor integrated circuit.